1. Field of the Invention
The present invention relates to an apparatus for forming deposited films with a microwave plasma CVD method in which the deposited films are formed on the surface of a substrate by introducing the microwave energy into a reactor vessel. More particularly, the invention relates to an apparatus for forming deposited films with a microwave plasma CVD method in which the deposited films are formed on a plurality of substrates simultaneously.
2. Related Background Art
For element members used for semiconductor devices, photoreceptor devices for electrophotography, line sensors for image input, image pickup devices, photovoltaic devices and so on, amorphous deposited films made of amorphous silicone compensated with hydrogen and/or halogen (e.g., fluorine, chlorine) have been proposed, and some of them have been put into practice.
Conventionally, a number of methods for forming such amorphous deposited films have been known, for example, a sputtering method, a heating CVD method in which a source gas is decomposed by heating, a photo assisted CVD method in which a source gas is decomposed with light, and a plasma CVD method in which a source gas is decomposed with plasma. Among them, the plasma CVD method, in which the source gas is decomposed by the direct current, the radio frequency, or the microwave glow discharge, deposited films like thin films are formed on a substrate made of, for example, glass, quartz, heat resistive synthetic resin film, stainless, or aluminum, has been widely used today for the formation of amorphous silicone deposited films for use in the photoreceptor for electrophotography. Especially in recent years, the microwave plasma CVD method using a microwave glow discharge decomposition has been noted for industrial purposes. The microwave plasma CVD method has an advantage of having a higher film deposition rate and a higher source gas utilization efficiency than other methods.
An example of the microwave plasma CVD method applying such an advantage has been disclosed in U.S. Pat. No. 4,504,518. This technology is to obtain good quality deposited films at a high film deposition rate by carrying out the microwave plasma CVD method at a low pressure of 0.1 Torr or less. Further, a technique for improving the utilization efficiency of source gas with the microwave plasma CVD method has been disclosed in Japanese Laid-Open Patent Application No. 60-186849. This technology is such that a plurality of cylindrical substrates are disposed to enclose an introduction portion for the microwave energy to form an inner chamber (or a film formation space), thereby greatly improving the utilization efficiency of source gas. In Japanese Laid-Open Patent Application No. 63-145781, there has been disclosed an apparatus for forming deposited films with an improved microwave plasma CVD method for forming functional deposited films. This apparatus for forming deposited films is such that a dielectric window through which the microwave is introduced into a film formation space is of a laminated structure, with a surface of the dielectric window facing a vacuum vessel being coarse, to improve the workability in exchanging this dielectric window, and to prevent the electrical resistance on the surface of films deposited on the dielectric window from decreasing, so that the microwave is prevented from reflecting at the surface of the dielectric window, thereby forming functional deposited films steadily at a high efficiency for a long term.
With these conventional techniques, it is possible to fabricate comparatively thick deposited films made of, for example, a photoconductive material at significantly great deposition rate and source gas utilization efficiency. An example of the apparatus for forming deposited films with such an improved conventional microwave plasma CVD method is shown in FIG. 4. This apparatus for forming deposited films is intended for the fabrication of photoreceptors for electrophotography. This point will be described below.
A reactor vessel 401 for the formation of deposited films is capable of evacuation, comprising an exhaust pipe 404 having one end thereof connected to exhaust means 406 such as a vacuum pump. An exhaust valve 405 is provided at an intermediate portion of this exhaust pipe 404. At central portions on an upper plane and a lower plane of the reactor vessel 401 are attached waveguides 403, respectively, each of which is connected via a stub tuner and an isolator, not shown, to a microwave power source, not shown. At an end portion of each waveguide 403 on the side of the reactor vessel 401 is disposed a respective dielectric window 402 to hold the vacuum within the reactor vessel 401. The dielectric window 402 is formed of a material (e.g., quartz glass, alumina ceramics) capable of transmitting the microwave power into the reactor vessel 401 efficiently and holding the vacuum airtightly.
A plurality of cylindrical substrates 407 are provided in parallel to each other, so as to surround a central portion of the reactor vessel 401. A space enclosed by the substrates 407 and the dielectric windows 402 within the reactor vessel 401 is a discharge space 408, within which a microwave plasma discharge is generated to form deposited films on the surfaces of the substrates 407.
Each substrate 407 is carried on a respective rotation shaft 419, which is connected to rotating means 412 such as a motor, wherein the substrate 407 rotates on its own axis when the rotation shaft 419 is driven by the rotating means 412. Also, each cylindrical substrate is provided with a heater 413 for heating the substrate so as to be inserted into the inside thereof.
Further, a source gas supply pipe 410 is provided to supply a source gas to the discharge space 408. The source gas supply pipe 410 is sealed at its top end portion, and provided with a number of holes on a lateral side of the top end portion, through which the source gas flows into the discharge space 410. The source gas supply pipe 410 is connected via a mass flow controller 409 to a source gas supply, not shown.
The operation of this conventional apparatus for forming deposited films with the microwave plasma CVD method will be described below.
First, a dry nitrogen gas or a dry argon gas at an atmospheric pressure is introduced into the reactor vessel 401, with the exhaust valve 405 closed, and then the reactor vessel 401 is opened and cleaned inside. Subsequently, cylindrical substrates 407 for the formation of deposited films are loaded into the reactor vessel 401. The substrates 407 are cleaned beforehand with an organic solvent such as trichloroethane to remove dirts such as oils or dusts on the surfaces. At the same time, the dielectric windows 402 sufficiently cleaned beforehand are attached to end portions of the waveguides 403 facing the reactor vessel 401.
Again, the reactor vessel 401 is closed and the exhaust valve 405 is opened to exhaust the reactor vessel 401, so that the internal pressure of the reactor vessel 401 is adjusted at a pressure of 1.times.10.sup.-7 Torr or less.
Then, the substrates 407 are heated up to a temperature suitable for the film deposition by respective heaters 413, while introducing an argon gas, for example, into the reactor vessel 401. And via the source gas supply pipe 410, a source gas such as a silane gas, for example, if amorphous silicon deposited films are formed, is introduced into the reactor vessel 401. In parallel with the introduction of the source gas, a microwave energy having a frequency of 500 MHz or greater, and more preferably 2.45 GHz, is introduced from a microwave power source (not shown) via dielectric windows 402 into the reactor vessel 401. By doing so, the source gas is excited and dissociated by the microwave energy in the discharge space enclosed by the substrates 407, so that the deposited films are formed on the surface of each substrate 407. At this time, the deposited films are formed around the entire peripheral surface of each cylindrical substrate 407 by rotating the substrate 407 around a central axis thereof along its longitudinal direction by each rotating means 412 such as a motor.
With such a conventional apparatus for forming deposited films it is possible to obtain deposited films having practical characteristics and uniformities at a certain deposition rate, and further to obtain deposited films with significantly less defects by cleaning the reactor vessel strictly.
However, in the conventional apparatus for forming deposited films with the microwave plasma CVD method, as above described, when forming large and thick deposited films requiring uniform characteristics, which are used for a photoreceptor for electrophotography, for example, there is still room for improvement on the aspects of the characteristics and the economical efficiency of deposited films formed.
That is, since films having a thickness equal to or greater than that of deposited films on the substrates may be deposited on the surfaces of dielectric windows or the source gas inlet conduit, in an apparatus configuration of the conventional apparatus for forming deposited films, parts such as dielectric windows or the source gas inlet conduit must be exchanged every time one formation of deposited films is performed. Therefore, to exchange these parts or take out the substrates having the deposited films formed, the reactor vessel must be exposed to the atmosphere every time, so that work such as evacuation, heating of the substrates, cooling after the formation of deposited films, atmosphere leakage, take-out of the substrates, and cleaning are required, and it takes a lot of time to do this work. Thus, there is a problem that the tactics, and thus the cost, for forming deposited films will be increased.
Also, there is a further problem that since the reactor vessel is exposed to the atmosphere every time, molecules such as nitrogen, oxygen or water in the atmosphere may be adsorbed to the inner surface of the reactor vessel to cause adsorbed molecules to be mingled into deposited films, thereby decreasing the film quality. Further, there is another problem that since the substrates are loaded into the reactor vessel in the atmosphere, dust already existing within the reactor vessel may fly up in a process of evacuating the reactor vessel after loading, thereby adhering to the surfaces of the substrates, and causing defects on deposited films.
Further, when the microwave power introduced into the reactor vessel was increased to raise the deposition rate, the temperature rose too greatly on the surfaces of dielectric windows on the side of the film formation space, so that the dielectric windows were sometimes broken.
Along with the rapid development of technologies of a copying machine, a laser beam printer and so on, it is requisite to obtain deposited films having more excellent characteristics and lower cost, and it is currently required for the apparatus for forming deposited films to form deposited films in a shorter time, with the improved characteristics of deposited films formed, and reduced defects.